Default constructed rosbag::ChunkedFile segfault on swap

[Hugal31]

rosbag::ChunkedFile::swap segfault if the ChunkedFile is default-constructed, because it tries to write to read_stream_->file_ while read_stream_ is null.

This causes rosbag::Bag to segfault as well when using std::swap, for instance when adding rosbags to a growing vector.

This code triggers the segfault:

#include <rosbag/bag.h>

int main()
{
  rosbag::Bag bag1;
  rosbag::Bag bag2;
  std::swap(bag1, bag2);
}